1. Technical Field
The invention relates generally to an improved suspension system for land vehicles. More particularly, the invention relates to trailing beam air suspension systems. Specifically, the invention relates to a lift mechanism for trailing beam air suspension systems.
2. Background Information
With the advent following World War II of large load carrying capacity trucks and trailers in this country, came the need to provide add-on axles for increasing the capacity of trucks over that of the chassis-cab design which is manufactured with a limited number of axles. While add-on axles effectively increase load-carrying capacity, it was soon realized that as the number of load bearing axles increased on a given vehicle, a number of difficulties arose. Specifically, tire scuffing, loss in fuel economy and the inability to safely corner, all work problems associated with add-on type axles. Mitigation of these problems was a primary concern to the industry, which concern results in the development of the liftable axle suspension system. Such a suspension system could be selectively raised from the road surface or lowered into engagement with the road surface when needed, thereby mitigating the aforementioned problems.
The transportation of goods by trucks continues to be a primary method of moving goods from one location to another. This commercial success is due to the large volume and load-carrying capacity available in standard trailers as well as the highway system which reaches virtually every part of North America. However, in order to assure that all manufactured trailers will travel easily on existing and newly constructed highways, trailer sizes have been standardized. Specifically, regulations have been passed which limit the trailer length, width and height. In an effort to increase the volume of the trailers, trailer manufacturers routinely dimension their trailers at the legal limit.
The use of large volume trailers amplifies the need for lift axles to be positioned under the trailer as the trailers themselves are able to carry more volume, and consequently, more weight. However, the distance between the trailer body and the road surface is relatively limited, and therefore lift axle suspensions must be manufactured to simultaneously permit the tire-wheel assemblies to move into and out of engagement with the road surface, while mounted to the trailer body.
Similarly, as the need continues to grow for the inexpensive and reliable transportation of goods, so does the popularity of road-railer suspensions such that the trailer may be supported on the trailer suspension, or alternatively on a fixed height rail bogey. A rail bogey generally includes a frame which supports at least two rail car wheels rotatably mounted on an axle. The upper portion of the frame includes a latching mechanism which is complementary related to a similar latching mechanism on the underneath of the trailer such that the trailer may be raised up via a road-railer suspension, positioned over the rail bogey, and lowered into engagement with the latching mechanism thereon. The axle of the road-railer suspension is then raised out of engagement with the railroad surface, with the rail bogey providing the suspension and wheels for use on railroad tracks.
Road-railer suspensions utilize a lifting mechanism which can either be an air spring, or a mechanical spring of the leaf or coil variety. The conventional axle lifting mechanism provides one or more stressed mechanical springs acting directly between the vehicle frame and axle. When air is relieved from the air springs, the mechanical springs raise the axle. The mechanical springs, in their condition of diminished stress when the axle is fully raised, must still exert sufficient force to support the weight of the axle and associated tires such that the tires remain in the raised position. When the air springs are pressurized, the wheels are forced downwardly into ground engagement overcoming the load forces and mechanical lift spring forces. In the road-railer application, the axle is moved between three separate positions: a first ground engaging position when the road-railer suspension is operating in the highway mode, a second ground engaging position, or transfer mode, when the trailer is raised to engage a rail bogey in coupling mode, and a rail mode where the trailer is supported on a rail bogey and the tires are lifted out of ground engaging position.
The lift mechanism must support not only the weight of the axle and wheels, but something greater than that weight in order to assure that if the rail suspension should encounter an irregularity in the track surface, the suspended tire-wheel assemblies do not operate under the increased force and bounce downwardly to come into contact with the track surface and cause significant damage to the suspension system and associated trailer. Still further, the spring must be of sufficient size to assure that as the spring deflection decreases, the spring force remains sufficiently high to retain the related suspension system in the chosen position. Hooks Law requires that as spring deflection decreases, so does the force provided by that spring. Conversely, as spring deflection increases, so does the force exerted by that spring. Prior art suspensions, while presumably adequate for the purpose for which they are intended, often provide much greater force at certain loci of the suspension travel than required and a minimum of force at other positions along the suspension path of travel. As such, the spring mechanism must be manufactured larger than necessary in order to provide the minimum required force to the suspension system at all locations along the suspension system path of travel. By so doing, the lift mechanism is necessarily larger and more costly than would otherwise be required if a constant force at an appropriate level was required at all loci along the suspension system path of travel.
More particularly, Hooks Law requires that as spring deflection increases, so does the force exerted by that spring. As such, a smaller more economical lift mechanism may be utilized if the suspension system provides a relatively constant force to the suspension at all loci along the suspension system path of travel. This need is especially important when a spring lift mechanism is utilized with a trailing or leading beam type suspension which travels along an arc thereby moving not only vertically, but translating longitudinally as it moves from a ground engaging to a non-ground engaging position. Still further, the longitudinal movement of the axle is substantially increased when a trailing beam is utilized in a road-railer application given the relatively long path of travel of the axle as the suspension moves from the coupling mode, when the suspension is fully inflated, to transport mode when the suspension is fully deflated and the tire-wheel assemblies are in a non-ground engaging position.
Leading and trailing beam type suspensions include a pair of longitudinally extending beams which may be either flexible or rigid, one of which is located adjacent each of two longitudinally extending slider rails located beneath the body of the truck or trailer. These beams are pivotally connected at one end to a hanger bracket extending downwardly from the frame, with an axle extending between the beams adjacent the other end. Additionally, an air or coil spring is generally positioned intermediate each frame rail and a corresponding beam. The beam may extend forwardly or rearwardly of the pivot, thus defining a leading or trailing beam suspension respectively.
Trailing beam suspension systems have been utilized for many years as they offer roll stability and may be tailored to create a roll flexible or roll rigid suspension system and are relatively simple to manufacture and easy to install. However, prior lift mechanisms utilized with trailing beam suspension systems have been difficult to install, and relatively expensive to manufacture given that the lift mechanism must operate through a relatively large longitudinal axle translation given that the trailing beam suspension rotates through an arc about a single pivot point.
The need thus exists for a lift mechanism for a suspension system or a trailing or leading beam suspension system which provides a relatively constant force through the entire path of travel of the axle at a force sufficient to support the suspension at every location along its path of travel. Additionally, the need exists for a lift mechanism which is simple to install and easy to manufacture.